The era of non-woven fabrics has not been and will not be the result of chance. Ever since man discovered that felt-like products could be made out of animal fibers, e.g., wool and fur, without weaving, there has been a continuing interest in trying to bond other fibers which do not naturally felt. Of course, a long hard look has been taken at papermaking processes wherein cellulosic fibrous materials, such as wood pulp, are inherently readily bonded into a dimensionally strong sheet. By simply beating cellulosic wood fibers, forming a dilute aqueous suspension thereof, and depositing same on a travelling wire-gauze screen or a rotating gauze-covered cylinder, paper sheets having excellent properties may be formed. Wet strength or water resistance is provided in the sheets by adding to the dilute aqueous suspension a small amount of synthetic resin having an affinity for the fibers.
Unfortunately, most of the natural and synthetic fibers presently enjoying great success in the woven fabric industry, such as cotton, cellulose esters, rayons, polyamides, polyesters, polyolefins, acrylics, and the like, do not exhibit this same inherent web-forming characteristic and cannot be beaten into a suspension. Consequently, a number of other methods have been developed, some practical and some not, for binding staple fibers thereof into a web having multi-directional strength.
Conventional carding equipment used in the weaving industry can produce fiber webs of uniform thickness suitable for impregnation with an adhesive or binder, but one drawback is that while lengthwise strength is usually good, cross-direction strength is generally not good at all, owing to the staple fibers being essentially parallel-laid, i.e., lengthwise of the fabric or in the machine direction of the material.
Random distribution of the fibers has been achieved by several methods, one of the most popular of which is that involving air-laying of the fibers by stripping same from a carded web by means of an air stream which then directs the fibers through a restricting throat which is controlled to adjust the thickness of the resulting web. The machine used for this purpose is called a "Rando-Webber" and was developed by a partly named Buresh for the Curlator Corporation.
A number of methods have been developed for treating randomly-dispersed webs with a binder. Typically, a water-based emulsion binder system is used in which a thermoplastic or thermoset synthetic polymer latex is prepared and a loose web of fibers to be treated is immersed therein, using special equipment in view of the structural weakness of the web; the thus treated web is dried and cured to effect proper bonding. Alternatively, an aqueous or solvent solution binder system of a thermoplastic or thermoset resin may be used to impregnate the fibrous web.
Still other methods include the application of thermoplastic or thermoset resin powders to the fibers, before or after making a web of same, and passing the web through hot rolls or a hot press to bind the fibers together. Alternatively, thermoplastic fibers having a softening point below that of the base fibers may be interpersed in a web of the latter and sufficient heat and pressure applied, such as by the use of heated rolls, to soften the thermoplastic fibers and bind the fiber network together.
Commonly used latices for non-woven fabrics are those prepared from polymers of butadiene-styrene, butadiene-acrylonitrile, vinyl acetate, acrylic monomers, such as methyl acrylate, ethyl acrylate, methyl methacrylate, and the like. While the emulsion binder system using latexes is the most popular method of forming non-woven fabrics, the homopolymers, copolymers and terpolymers heretofore used therein have suffered from several or more shortcomings. Since, for example, the end uses to which the non-woven fabrics are put play a major role in determining what polymeric binder is used, it can readily be appreciated that the properties of the polymeric binder are critical. Among the many tests to which non-woven fabrics are subjected are those which determine wet strength, washability, ability to hold up under repeated dry cleaning conditions, color fastness, hand, drape, abrasion resistance, resiliency, etc. Of course, the results of these tests will vary considerably depending upon the base fiber employed, let alone the combination of same with a binder.
Always a real consideration in the selection of binders is the cost thereof. As is well known, non-woven products are presently used, to name a few, as interlinings, wiping cloths, mops, shoe innersoles, book bindings, backings for plastic sheets, liquid filters, sanitary products, ribbons, diapers, battings, insulation, etc. Obviously, the cost of the binder must be consistent with the end use intended for the non-woven fabric. Needless to say, however, as the end uses become more varied and more sophisticated, it can be expected that the price range will react accordingly.
While the acrylic polymer latices hereinbefore mentioned are presently enjoying significant success, it is no secret that the cost thereof is a drawback. Consequently, there is still a real demand for a versatile, effective binder which is not only attractive from a cost standpoint, but which is, for example, capable of bonding fibrous materials into non-woven fabrics and rendering same strong, durable, resistant to water and dry cleaning solvents, soft to the touch, etc. The present invention is directed to these and related goals, as will be more fully understood from the description of the invention which follows.
The following United States patents are deemed of interest: Nos. 3,301,809, 3,380,851 and 3,451,982.